Fluke 8050A LCD
display replaced with LED seven-segment

Recently
I was able to cheaply buy a second hand Fluke 8050A made in the early
eighties. It is a nice 4.5 desktop digit meter, but a typical problem
seems to be that the LCD wears out and goes black. Replacement LCD
displays are hard to find, not cheap and probably lead to similar
problems in due time.

The LCD
display is interfaced by standard components (CD4054 and CD4056) that
drive each segment individually. This allows it to be replaced with a
LED 7-segment display. However LCD displays are driven
normal/inverted about 30 times/second. LED displays can be used by
disabling this clock and connect it to a fixed level (high).

Since
my Fluke does not have the battery option I was pretty confident that
the power supply would be able to provide the additional LED current.
The LCD drivers are able to sink current per segment in the order of
2mA. Sinking capability is somewhat higher than sourcing, so I
decided on an LED display with common anode.

Since I
had no idea how bright a LED display is with this amount of current,
I ordered the brightest 9mm displays I could find. If they turned out
too bright I could always reduce the current. And that is what
happened. In the end a series resistor of 2k2 showed proper results.
With 1k the display was brighter, but some bleeding to neighbouring
segments started to become noticable.

The LCD
display also has a couple of special symbols for high-voltage(HV),
low battery(BT), decibel (dB) and relative measurement (REL). Of
course these have to be handled differently and I simply printed them
as inverted text on paper. The paper is then lit from behind with
separate high bright LEDs. I blackened the laserprinter output a bit
more with a felt pen.

Another
special case is the plus (+) and minus (-) of the LCD display.
Personally I do not see the need for a plus symbol, so I decided to
use the minus only since the LED can provide this nicely. The most
significant digit segment g is used for this purpose. To “disconnect”
it from the digit I tipped a small peace of the segment with a black
CD felt pen.

However
the behaviour of the +/- signals is a bit strange. I have no
knowledge of how this worked originally on the Fluke. After playing
around what seemed to work in all measuring modes (R/V/A/dB and REL
combinations) is to use the inverted '-' signal and disable the
segment when '+' is high (??!!). Without using the '+' signal,
measuring ohms has the minus sign always on. Oh, well, this can all
be done using a single small BS170 mosfet.

Below
is shown the detailed images of the whole process, the circuit
diagram of the display board and the circuit modifications.

The new
LED displays in stages

The
bad LCD display

Replaced
it with 9mm LED displays and 3 LEDs

With
printed symbols on paper

Paper
is made more black with felt pen

The
selected LED display details:

Kingbright
SA36-11SRWA

Red 9mm
display

Common
anode

Brightness
2200-9000 uCd @ 10mA

Conrad
order code: 158780

Dimensions:

The
Fluke 8050A display board circuit (photo)

Circuit
modifications:

Note
that each individual segment has a 2k2 resistor between it and the
driver pins. I soldered the cut resistors directly to each driver IC
and wired them from there to each segment. This was a bit more work
then I expected... The symbol LEDs have 1k in series. For a Fluke
with the battery option you would need an additional LED.

The
results:

Further
ideas RS-232 interace:

Now I
understand the circuit en have seen the signals on an oscilloscope, I
was thinking of making an isolated RS-232 interface to this meter. It
is a desktop model that is mains powered, so keeping it switched is
not a problem. This would allow long duration measurements like
rechargable battery charge/discharge curves etcetera. To isolate the
circuit to be measured from the computer would also require an opto
isolated interface. It might never happen, but if it does I will
update this page.